Nuclear reactors cooled by pressurized water generally comprise a vessel of cylindrical shape placed, in service, with its axis vertical and having a bottom end constituted by a round bottom secured to a bottom end of the cylindrical wall of the vessel, and a top end constituting a support flange for a head which is generally of hemispherical shape and which can be secured to the vessel in a manner that is proof against the pressurized water contained in the vessel while the nuclear reactor is in operation. After the primary circuit of the nuclear reactor has been cooled and depressurized, the head can be dismounted in order to give access to the inside of the vessel which contains the core of the nuclear reactor.
In general, the reactivity of the nuclear reactor core is adjusted by means of control rods of absorbent material which are moved vertically inside the nuclear reactor core. The nuclear reactor control rods are secured to the bottom ends of drive shafts that pass through the vessel head inside tubular adapters that are of generally cylindrical shape and that have mechanisms secured thereto for moving the control rods in the vertical direction.
While the nuclear reactor is in operation, temperature measurements are taken inside its core by means of columns of thermocouples which likewise pass through the vessel head inside adapters.
The vessel head thus has a plurality of cylindrical tubular adapters passing through it, each being secured to the inside of a bore having a vertical axis (when the head is in its in-service position on the vessel), with the various adapters being distributed in a plurality of rows and in a plurality of zones of the head that are annular about the vertical axis that is common to the vessel and the head and on which the center of the spherical wall of the vessel head is situated. Depending on their positions through the vessel head, the bores through which the adapters pass (which are all parallel to the axis of the vessel head), themselves have axes at various acute angles relative to the radii of the hemispherical head passing through respective points on the axes of the bores. In particular, one of the annular rows of bores passing through the vessel head is disposed in such a manner that the axes of the bores are at an angle of about 38° relative to the corresponding radii of the hemispherical vessel head. In general, the bores have axes that do not pass through the center of the hemispherical head (with the exception of one bore that is placed on the vertical axis of the vessel), and the intersections between the bore of cylindrical shape with the outside and inside walls of the vessel head present shapes that are complex.
Each adapter passing through the vessel head presents both a top portion that projects above the vessel head, said portions having secured thereto, in particular, the mechanisms for moving the control rods, and a bottom portion that projects beneath the vessel head, which bottom portion is shorter relative to the inside surface of the head than is the corresponding top portion, and serves in particular to receive a cone for re-engaging a drive shaft.
The adapter tubes are generally made of a nickel-based alloy such as 690 alloy, and the vessel head is made of a low-alloy ferritic steel and coated on its inside surface in a layer of stainless steel. The adapter tubes must be secured in their bores passing through the head in such a manner as to be completely proof against the pressurized water that fills the vessel while the nuclear reactor is in operation (at a temperature of about 320° C. and at a pressure of about 155 bars) and they must be capable of withstanding the pressure inside the vessel.
The adapter tubes are engaged tightly in the bores passing through the vessel head and they are secured by being welded to the inside portion of the vessel head which is constituted by low-alloy steel coated in stainless steel. In each of the zones of the inside portion of the head in which a passage is made for an adapter tube, an annular facing is machined to surround the bore through which the adapter tube passes, and a welding material that is metallurgically compatible with the material of the adapter tube is deposited in the facing by welding (generally by melting a wire). Thereafter, the head is drilled to form the bore for passing the adapter, the adapter is secured tightly in the bore, and finally the adapter is welded by depositing a welding material in a portion of the facing around the adapter, in order to secure the adapter to the previously-deposited layer of welding material.
The operation of depositing a layer of welding material in the annular facing prior to drilling the bore is generally referred to by the term “buttering”.
Until now, the operations of initially depositing a first welding material in the annular facing prior to drilling the bore, and of welding the adapter tube by depositing a second welding material in the remaining portion of the facing after the bore has been drilled and the adapter tube mounted therein, have been performed manually, in particular because of the complex shape of the connection surfaces between the adapters and the inside portion of the vessel head.
Such operations are lengthy and expensive and require numerous inspections, since the welding must be free from defects. The number of adapter tubes secured to a vessel head is generally large (e.g. 65 or 77 adapter tubes, depending on the type of nuclear reactor), which makes this operation extremely lengthy and expensive.